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Following the Paris attacks in November, in which Isis (Islamic State) gunmen killed 130 people in a series of mass shootings and suicide attacks, Brussels raised its terror threat level to its highest possible alert.

The Belgian capital also went on lockdown, as police and intelligence agencies traced at least one of the Paris attackers' origins back to the Molenbeek neighbourhood of Brussels.

Roughly a week later, the Belgian Government lowered the threat level from "serious and imminent"to "possible and likely"where it has remained until the latest attacks. It is now back to its highest level.

"This is the ultimate paradox in counter-terrorism," said Bruce Hoffman, the director of Georgetown University's Security Studies Programme. "You can't protect every target, everywhere, all the time. They've been on complete alert, and still all these measures are still insufficient against a determined adversary."

The attacks hit a city where soldiers, clad in body army and Fabrique Nationale carbines were already stationed at metro entrances, government buildings and various other locations throughout the city. The soldiers often patrol in pairs and are occasionally easily marked by their large Mercedes transport trucks.

The blasts tore through the departures area at Brussels airport, which is regularly patrolled by soldiers but does not have a screening area for passengers.

After exiting the metro or inter-city trains arriving at the airport, passengers walk to an atrium where a three-level lift takes them either to the departures or arrivals area. To enter the departure area, passengers only have to scan their boarding passes before proceeding to security.

At rush hour, these areas, including the ticketing desks, can easily back-up, something the attackers clearly took advantage of as one of the bombers appeared to have detonated their device in exactly that vicinity.

In the Brussels metro however, the only security is the occasional police or group of soldiers at various entry points.

"This is what makes planning for these attacks so difficult," Hoffman said. "You might place security at the central train station but there are still a cornucopia of different areas that are open to attack across the city." In 2011, a suicide bombing carried out by Islamic militants at the Domodedovo International Airport bombing used similar tactics - striking in the arrivals hall where there were no security checkpoints.

The explosions in Brussels comes just days after Belgian police arrested Salah Abdeslam, one of the suspected participants in the Paris attacks. The 26-year-old allegedly planned logistics for the cell that carried out the deadly siege in November and was likely a key leader in the group.

According to Hoffman, the attacks just four days following Abdeslam's arrest, indicates how entrenched his compatriots are in the region.

"Their leader was picked up and the network didn't crumble, they adjusted their plans and responded in in a pretty expeditious fashion," Hoffman said, though it is currently unclear if Abdeslam was directly connected to the new blasts. "It indicates their network and infrastructure is much deeper than people have assumed."